The Cedrus response box is connected to a PC presentation stimulus computer via the USB connector (we use a USB extension cable). The stimulus presentation software that we use is Neurobehavioral Systems Presentation which takes as input the response pad and outputs to the EGI system via a parallel port that goes through an EGI AV device to a serial port. This serial port is connected to the EGI AMP that then through a fiber optic cable goes to a switch box to an ethernet cable to the EEG acquisition computer.

The fact that this happens when using an external keypad as well makes me think that the issue is not with the response pad, but rather elsewhere with the PC or the cabling.

A potential source of trouble might be the USB extension cable. How long is it?

I ask because the USB standard is a “desktop” bus, meaning that it was designed for devices that can be found on your desktop. The length of USB cables is limited to about 10 feet. Beyond that, you will need to use a powered USB repeater. This is a device that receives the USB signal and amplifies it so it can travel further.